gb_proxy keeps stale PTP-BVCI <-> NSVCI mappings

When BVCI A first establishes a connection through NSVCI/NSEI B, then later establishes a connection through NSVCI/NSEI A, gb_proxy permanently keeps a stale proxy mapping associating BVCI A with NSVCI B.

This causes all downlink messages from the SGSN to be routed still through NSVCI B, despite now NSVCI A being the correct one.